Amplifying system



Aug. 14, 192s. 1,680,424

W. A. MacDONALD AMPLIFUNG SYSTEM Filed Feb. 3, 1926 GJ A l l ORN EYS Patented Aug. 14, 1928.

UNITED s'rATr-:s

PATENT OFFICE.

WILLIAM A. MACDONALD, 0F LITTLE NECK, NEW YORK, ASSIGNOB TO HAZELTINE CORPORATION, 0F JERSEY CITY, NEW JERSEY, A CORPORATION OP DILAWRE.

AMPLIFYIENG SYSTEM.

Application led February 3, 1926. Serial No. 85,815.

This invention relates to amplifying systems, and especially to high-frequency amplifying systems wherein several vacuum tubes are connected in cascade, and coupled by means of hi hly eilicient circuits.

The object of the invention resides in the provision of means for increasing the stailit of such systems without sacrificing the esired amplification; and, concurrently, for effecting a more uniform voltage ampliication characteristic throu hout the operating frequency range of t e system. To this end, the impedance of the input circuit of one or more of the amplifying tubes is reduced to a suitable value, while the average impedance of the preceding circuit coupled-thereto is increased so as more nearly to e ual the impedance of the circuit from whic it derives its ener The present invention is especially applicable to radio receivers provided with means Vfor neutralizing the undesirable capacitive coupling between the input and output circuits of the high-frequency amplifying tubes, such as the arrangement described'in Hazeltine U. S. Patents Nos. 1,489,228 and 1,533,858. In the operation of radio receivers wherein highly efficient tuned circuits are employed with a view toward obtaining maximum high-fre uency amplification per stage, it has been ound that the tendency toward undesirable oscillations due to stray feedback couplings between portions of the various input and output circuits is especially noticeable. For example, the extremely small capacitive couplin existing between the detector input trans ormer and the antenna coil, while usually negligible in amplifiers utilizing one or tivo stages of high-frequency am ilification of moderate magnitude, may be su cient to cause undesirable oscillations, or tendency toward oscillations with accompanying si al distortion, in the case of amplifiers uti izing maximum amplifica-- tion per stage, especially when three or more stages are employed. The filament system' being common to the several tubes, a stray or undesirable coupling, either capacitive or electromagnetic, or oth, between the input circuit of the first high-frequency amplifying tube` of the series and some subsequent portion of' the receiver, such as the input circuit of the detector tube, will cause a potential diii'erence across the input-circuit impedance of the first high-fre uenc am lif in tube above-mentioned. Sqince the gid-dilagment electrodes of the first tube are also termi- A nals of the input-circuit impedance, this potentialdierenee will impressa voltage upon the grid of that tube and hence the undesirable feed-back voltage thus impressed upon the grid of the first tube will be proportional to the value of impedance between the grid and filament terminals. This impedance will hereinafter be referred to as the impedance of the grid circuit or inputcircuit impedance. It is Well understood in the art that in any given amplifyin cir` cuit arrangement including vacuum tu es, a certain amount of feed-back of energy can be tolerated before undesired oscillations are produced. However, for best results, it is preferable to eliminate substantially all undesired feed-back which tends toward instability even when such feed-back is of a degreeinsuiiicient to produce oscillations. Accordmgly, it is the purpose of the present invention to provide a method and means whereby this input-circuit impedance of whatever tube in the series is thus undesirably effected, ma be reduced to a value sufiiciently low to su stantiall eliminate the undesired effects of feed-baul; voltage upon the grid of that tube Without substantially reducifng tlie effective desired voltage amplification of the system; and concurrently to realize a more uniform voltage amplification characteristic thruout the operating frequency range of the system.

Experimental measurement has shown that with the highly etlicient timed coupling transformers preferably employed in amplifying systems for broadcasting pui'- poses, such as, for example, one having am operating frequency range of 3 to 1, 1. e., 1500 kilocycles to 500 kilocycles, a voltage amplification characteristic of approximately to l to 21/2 obtains when these transformers are cnilployed ir conjunction with a vacuunz tube iai/ing a mutual conductance of aboutL 1000 micrombos. Under suoli conditions, tlie voltage amplification at 1500 kilo- Y Cycles may be approximately 16, with :i gradually decreasing amplification gradient to 500 irilocyeles. It is evident, therefore, that when tivo stages, similar to that just described, are connected in cascade, the amplification at 1500 kilocycles will be 16, or

fili t, while at 500 kiloeyeles the amplification v-.itl lie (3.41 or 4l.. Accordingly. if three .sach etages are operated in iascade, the amplilic; tion will vary as the cube. and so on. Similar conditions obtain in the, input circuit ol' the, first stage of a. multistage amplifying; system supplied with energy from an antenna, whether it be of the open type or of the loop type. In order to .secure `satisfactory selectively willi such a system empioyini,r an open type antenna, it is usuali)v desirable to include between the antenna and the iniiut circuit of the lirst amplifying stage a step-up ratio of voltage transformation greater than 2 to l. To attain this ratio, the untuned antenna coil which is cou pied to the input circuit of the first highfrequency amplifying stage is. usually composed of a Small number of turns. ThisV `small iuduetance. together with the natural capacity of the uuml antenna system eniplojved in broadcast reception, provides a circuit resonant at a rather high fnudamen tal fretpiency. and. under ordinary circumstances, such a circuit would likely be resonant at. a frequency of approximately ltltl kilocycles, thereby providing; maximum signal energy and amplitication aty that fretptenev. with a decreaqiug amplilication gradient, toward the tower frequencies.

i The attainment of greater uniformity of amplification over the operating frequency range merely hy increasing the secondary-toprimary transformer voltage ratio has4 heretofore been impractieable, because of the resulting high amplification, with its colmequent tendency toward instability and oscillation. Vfhen the capacitive coupling between the input and output circuits of each tube has already been neutralized, such oscillations are `generally Caused by a feedhacli of energy between non-adjacent circuits, due to the Ltray capacitive or magnetic couplings between them. This tendency toward osciliation is increased. as previously expiaiued. by the relatively high input impedance of the coupling transformers, which tends to force feedback voltages through one or more of the vacuum tuhes by impressing undesirable potential differences between the lgrid eiectrodes thereoef and the filament system.

ThisA invention provides means for olitaining a greater uniformity of amplication throughout the operating` frequency range of the amplifying system. while removing a prirafipal cause of instability and undesirable oscillations. The invention may more readily be understood by a consideration ot' the foliowin Y description of a preferred e|nhodimeut thereof as illustrated in the drawv ing. which shows a circuit diagram of a` radio receiver includingV four vacuum tubes V, to VV inclusive; the first two being highfretluency amplifiers, the third a detector. or rectifier, and the fourth a low-, or audiofrequency amplifier. The capacitive rou plint: between the input and output circuits of vacuum tubes V1 and VH, respectively, is herein intended to be neutralized in act-ordance with the method described in the men tioned ilaaeltine patents. For this purpose. neutralizing eondensers CM. (fm2, are pro vided, each of which is connected troni the grid of the tubes Y, and V2, respectively, to a. suitable point on the secondary coil of the succeeding high-frequency amplifyingr transformers T2 and '13. The input circuits of tubes V1, V2 and VL are tuned by means` of adjustable condensers U1, C: and GJ. rthe usual lilament current control rheostats are symbolically indicated, as are also suitable batteries vfor supplying operatitng potentials to the lilaments, and to the plate and grid electrodes ol the tubes. A loud speaker, or other low-lreipiency translating device: L5., is' indicated as connected in the output circut. olI they last low-frequeney, or audio. am plit'yiug tube V4. A by-pass condenser Cm, is connected between the common terminals of the primary coils of coupling transform4 ers T! and 'lla andthe filament system, inorder to provide a ltnv-impedance path for the hiuli-frequency return from output circuits to the lilament system.

While the receiver iiluatrated includes but two higlrfrequency amplilication stages, it should he understood that as many more such stages as necessary may he added; and. accprdiug to thc preceding' explanation. il. will be plain that theI present invention he comes of increasing importance as the number of high-frctptency stages is increased. It should also be noted that as many stages ot' low-frequency amplification al desired may be employed in the well-known manner The first high-frequency amplifying tuhe V1 is Coupled to the antenna system A. hy means of antenna-input circuit coupling coil T1, which may, in the alternative. comprise a two-Winding transformer. Coil T1 is tuned by :in 'adjustable condenser C1, 'connected across its terminals. Accordingr to customary practice, this` coupling coil would be connected to the grid g1 of tube V1 at the extreme. terminal l of the coil. In like manner the grid connection q2 of tube V2 would he eormected to the upper terminal 7 of the Secondary coil of transformer T2. However, it has been found. as explained above. that when such connection is made, the impedance of the coil between terminals 4 and IB (or, in the ease of transformer T2, between the terminals T and 8) is usualiy suliieiently high to impress` undesirable voltages of appreeiahle value upon the input circuit of the tube; such voltage being presentr because of stray capacitive or magneticv coupling between the input circuit of tube V, (or tube Y!) and `some subsequent highfrequeney circuit, such as the output circuit itil* Vz, the input circuit of Va, or some portion of a succeeding high-frequency stage, in case more than two such stages are employed.

Because of the elusive character of such stray couplings, it is frequently impracticable to eliminate them, but the disadvantageous eii'ects thereof may, in accordance with the present invention, be minimized by suitably reducing the impedance of the input circuit of the tube thus undesirably affected. In the case of tube V1, the grid connection is tapped on to antenna-coupling inductance T1 at some point substantially below the highest-potential terminal 4 of the coil. For the sake of simplicity, we may assume that this oint is midway between the extremities of t e coil, although in practice it is sometimes preferable to locate the tap elsewhere along its length. It follows, then, that the eii'ective transfer of energy between one circuit (A, 2, 3, C) and the succeeding circuit (g1, 1, 3, f1) will be reduced. By thus placing the grid connection upon the midpoint 1, of the input inductance T1, the impedance of the input circuit between the grid and the filament is reduced to approximatelA one-quarter of the value to be obtaine by connecting the grid to the extreme terminal 4. This midpoint connection, of course, also reduces the desired voltage upon the grid of tube Vl due to incoming signals, but this voltage is not reduced in proportion to the reduction of the imped-` ance; measurements indicating that the signal voltage is thereby reduced a considerably smaller amount than is the impedance. In order to restore the signal voltage to a satisfactorily high value, the voltage across the input circuit inductancer 1, 3 may be increased by locating the antenna tap 2 nearer the inductance terminal 4, at such a point that the required si nal voltage is obtained upon the grid of tu e V1. It is clear that, once the proper number of primary turns is determined, the location of the tapped connection` 2 may be made permanent for all other amplifiers similarly constructed. This increase in turns in the antenna inductance provides in addition the previously suggested advantage of decreasing the resonant requency of the antenna circuit, the result being an increase in the amplification toward the low-frequency portion of the tun- Aing range due to a nearer approach to resonance at that ortion, thus improving the uniformity of t e overall voltage amplification characteristic of the whole amplifier.

In the event that a loop antenna is employed instead of the open antenna as illustrated, the tuning condenser C1 would be connected across the extremities of the loop winding; the midpoint, or other intermediate point on the loopbeing connected directly to the grid ,gy the switches 9 and 10 be opened, thus disconnecting the open antenna and ground,

the inductance T1 with its tuning condenser C, may be considered to represent a tuned loop antenna system. When used as a loop, the inductance T1 would usually be physically larger than when employed as a coupling coil for an open antenna, as illus trated. In both cases, however, the circuit dia'lglram representation is the same.

e application of this invention to transformer T2 introduces similar improvements in the second stage of amplification, where a similar process may be followed. For example, the connection from the grid lead g, may be placed at the mid oint 5 of the secon ary winding 7, 8 o transformer T2, thereby reducing the average grid-filament or input circuitimpedance to about onequarter of the value obtained by connecting the grid lead to terminal 7; and concurrently restoring and maintaining the desirable signal voltage upon the grid of tube V2 ny 1ncreasing the voltage across primary Winding 6. This voltage increase may conveniently be obtained, as before, by increasing the number of turns included in the primary winding 6 to a suitable value. The increase in primary turns introduces the additional advantage of causing the average impedance of the primary circuit more nearly to equal the output circuit impedance of the preced- 4 t ing tube, namely V1. As is well known in the art, such matching of impedances results in a more favorable voltage amplification ratio, as evidenced by the attainment herein of a nearly uniform voltage amplification over the operating frequency range of the system.

For the reason that in a system such as that described, the impedances can actually be matched at only one frequency, it is advantageous so to arrange the input and out put circuits that the respective impedances are matched at that intermediate frequency of the entire operating frequency range of the system which will result in t ie particular voltage amplification characteristic desired. By Way of example, the system herelill in described would a proximate an horiprovement is introduced by the application of this invention to vacuum tube amplifiers whose effective grid-plate capacity hasbeen neutralized, inasmuch as a greater tube capacity tolerance is thereby allowable; i. e., when the input-output circuit capacities in an aniplifyin system have once been neutralized or ba anccd to prevent undesirable oscillations, the same neutralizing adjustments may be retained even when there are substituted other vacuum tubes having somewhat different grid-plate capacities, without causing the system to become unstable.

The foregoing description of a single embodiment of the present invention necessarily concerns a particular form of radio apparatus, but it 1s to be understood that the principles described herein, and subsequently claimed, are intended to be applicable with equal force to any form of receiving or transmitting apparatus in which efficient high-frequency amplification is desired,

` I claim:

1. In a multi-stage high-frequency vacuum tube amplifier system havin coupling between two stages thereof where y a tendency toward undesirable oscillation is produced in the tube of at least one stage of said system, the method of minimizing the effects of said cou ling without substantially reducing the desired voltage amplification of the system, which comprises reducing all fluctuating voltages impressed upon the -grid -1 electrode of the tube thus undesirably affected, by reducing the impedance of the input circuit of that tube, and decreasing the step-up ratio between said input circuit and the output circuit of the vcauum tube next preceding, so that the desired signal voltage fluctuations upon said grid are maintained.

2,111 a multistage high-frequency amplifier system including transformers coupling adjacent stages, each stage including a vacuum tube, undesirable coupling between the input'circit of the vacuum tube in one stage and a portion of a succeeding stage whereby undesirable feed-back: voltage is impressed upon the-,grid electrode of said last-mentioned vacuum tube, a tuning condensenconnected across the extremities of the secondary winding of a coupling transformer included in said input circuit, and a connection from the grid electrode of said vacuum tube to a tap intermediate the extremities of said secondary Winding, whereby the feed-back voltage impressed u pon said grid is reduced in greater proportion than the signal voltage.

3. In a tuned multi-stage high-frequency amplifying signalin system, each stage including a vacuum tu e and a coupling transformer comprising a primary winding and a secondary winding, said secondary winding having a certain maximum inductance value and being tunable over an operating frequency band, said s stem havinov coupling between two stages tllereof wherdby a tendency toward undesired oscillation is produced by the tube in whose input circuit said secondary winding is connected, the method of vminimizing the effects of said coup ing without substantially reducing the desire Signal voltage amplification of the system W iicli comprises connecting substantially less than the maximum inductance value of said secondary winding in said input circuit whereby the input circuit impedance is decreased, and increasing the impedance of said primary windin to approximatel match the im cdance 0% the circuit in Whic it is connecte whereby the normal value of signal voltage impressed upon the grid of said last-mentioned tube is substantially restored.

4. In a multi-stage high-frequency amplifier system including transformers coupling adjacent stages, cach stage including a vacuum tube, undesirable coupling between the input circuit of the vacuum tube in one stage and a portionrof a succeeding stage whereby undcs'rable feed-back voltage is impressed upon the grid electrode of said last-mentioncd vacuum tube, a tuning condenser connected across the extremities of the secondary Winding of a coupling transformer included in said input circuit, the primary winding of 'said transformer being connected in the output circuit of a preceding vacuum tube, and a connection from the grid electrode of said vacuum tube to a tap intermediate the extremities of said secondary winding whereby the feed-back voltage impressed upon said grid is reduced in greater proportion than the signal voltage.

5. In a radio receiving system including an antenna system, a multi-stage high-fre quency vacuum tube amplifier, one stage of which is linked with said antenna system by means of a coupling device comprising a primary Winding connected in the antenna system and a secondary winding connected in the input circuit of said stage, whereby signal vo tages are impressed upon the grid electrode of the vacuum tube of said stage, undesirable coupling between the input circuit of the vacuum tube in said stage and a portion of a succeedin stage whereby undesirable feed-back voa s are impressed upon the grid electrode o the vacuum tube of said stage, capacity means connected across said Isecondary winding for tunin the same to the frequency of said signa voltages within an operating frequency band, said primary winding having an inductance value such that the natural period of said antenna system is of a frequency less than the maximum frequency of said band and greater than an intermediate frequency of said band, and a connection from said grid lou Iglo

to a. tap on said secondary winding intermediate the ends thereof whereby the impedance of said input circuit is iimited to a value less than the total impedance of said secondary winding so that said feed-back voltages are reduced in greater proportion than said signal voltages.

6. In a radio signaling system, a multistage high-frequency amplifier including transformers coupling adjacent stages, each stage including a vacuum tube, undesirable coupling between the input circuit of the vacuum tube in one stage and a portion of a succeeding stage whereby undesirable feedback voltages are impressed upon the grid electrode of the vacuum tube of said stage, a tuning condenser connected across the extremities of a secondary winding of a coupling transformer included in Said input circuit whereby said circuit may be tuned over an operating frequency band, the primary winding of said transformer being connected in the output circuit of a preceding vacuum tube and having an effective value of impedance substantially the same as the plate impedance of said preceding vacuum tube as measured at a frequency higher than the minimum frequenc of said operating band and lower than the mid-frequency of said band, and a connection from the gridelectrodc of said first-mentioned vacuum tube to a tap intermediate the extremities of said secondary winding whereby the feedback voltages impressed upon said grid are reduced in greater proportion than the signal voltage, and the signal amplification of said amplifier is maintained at an approximately constant value thruout said operating frequency band.

7. In a radio receiving system including an antenna system, a multi-stage high-frequency vacuum tube amplifier, one stage of which is linked with said antenna system by means of a coupling device comprising an antenna inductance connected in the antenna system, and a secondary inductance connected in thel input circuit of said stage, whereby signal voltagesare impressed upon the grid electrode of the vacuum tube of said stage, undesirable coupling between the input circuit of the vacuum tube in said stage and a portion of a succeeding stage whereby undesirable feed-back voltages are impressed upon the gridvelectrode of the vacuum tube of said stage, capacity means connected across said secondary inductance for tuning the same to the frequency of said signal voltages within an operating frequency band, said antenna inductance having a value such that the natural periodof said antenna system is of a frequency less than the maximum frequency of said band and greater than au intermediate frequency of said band, and a connection from said grid to a point on sigd secondary inductance intermedizitr the ends thereof whereby the impedanee of said input circuit is limited to a value less than the total impedance of said secondary inductance so that said feed-back voltages are reduced in greater proportion than said signal voltages.

In testimony whereof I affix my signature.

WILLIAM A. MACDONALD.

CERTIFICATE 0F CGRRECTION.

Pltent No. l, 680, 424.

WILLIAM A.

Granted August 14, 1928, to

MacDONALD.

It is hereby certified th at error appears in the printed s ec' t icati shove numbered patent requiring correction as follows: Page pi, l ine 9g" iiifriiiliee out the word "to" first occurrence; page 2,

rend "selectivity"; same page,

the case `In the Patent Office.

line 10, for the word "selectively" lilie 82 for the miss u i Delled word read operxting line 93. after the word "from" insert the word opel-umn:

line'l30, after the word "circuit" second occurreii 3, ine 2l. for the letter "C" read "G"; and th read with these corrections therein that the sam "both", and ce insert the word "of"; page e said Letters Patent should be e may conform to the record of Signed and sealed this 23rd day of October, A. D. 1928 (Seal) M. J. Moore, Acting Commissioner of Patents.

to a. tap on said secondary winding intermediate the ends thereof whereby the impedance of said input circuit is iimited to a value less than the total impedance of said secondary winding so that said feed-back voltages are reduced in greater proportion than said signal voltages.

6. In a radio signaling system, a multistage high-frequency amplifier including transformers coupling adjacent stages, each stage including a vacuum tube, undesirable coupling between the input circuit of the vacuum tube in one stage and a portion of a succeeding stage whereby undesirable feedback voltages are impressed upon the grid electrode of the vacuum tube of said stage, a tuning condenser connected across the extremities of a secondary winding of a coupling transformer included in Said input circuit whereby said circuit may be tuned over an operating frequency band, the primary winding of said transformer being connected in the output circuit of a preceding vacuum tube and having an effective value of impedance substantially the same as the plate impedance of said preceding vacuum tube as measured at a frequency higher than the minimum frequenc of said operating band and lower than the mid-frequency of said band, and a connection from the gridelectrodc of said first-mentioned vacuum tube to a tap intermediate the extremities of said secondary winding whereby the feedback voltages impressed upon said grid are reduced in greater proportion than the signal voltage, and the signal amplification of said amplifier is maintained at an approximately constant value thruout said operating frequency band.

7. In a radio receiving system including an antenna system, a multi-stage high-frequency vacuum tube amplifier, one stage of which is linked with said antenna system by means of a coupling device comprising an antenna inductance connected in the antenna system, and a secondary inductance connected in thel input circuit of said stage, whereby signal voltagesare impressed upon the grid electrode of the vacuum tube of said stage, undesirable coupling between the input circuit of the vacuum tube in said stage and a portion of a succeeding stage whereby undesirable feed-back voltages are impressed upon the gridvelectrode of the vacuum tube of said stage, capacity means connected across said secondary inductance for tuning the same to the frequency of said signal voltages within an operating frequency band, said antenna inductance having a value such that the natural periodof said antenna system is of a frequency less than the maximum frequency of said band and greater than au intermediate frequency of said band, and a connection from said grid to a point on sigd secondary inductance intermedizitr the ends thereof whereby the impedanee of said input circuit is limited to a value less than the total impedance of said secondary inductance so that said feed-back voltages are reduced in greater proportion than said signal voltages.

In testimony whereof I affix my signature.

WILLIAM A. MACDONALD.

CERTIFICATE 0F CGRRECTION.

Pltent No. l, 680, 424.

WILLIAM A.

Granted August 14, 1928, to

MacDONALD.

It is hereby certified th at error appears in the printed s ec' t icati shove numbered patent requiring correction as follows: Page pi, l ine 9g" iiifriiiliee out the word "to" first occurrence; page 2,

rend "selectivity"; same page,

the case `In the Patent Office.

line 10, for the word "selectively" lilie 82 for the miss u i Delled word read operxting line 93. after the word "from" insert the word opel-umn:

line'l30, after the word "circuit" second occurreii 3, ine 2l. for the letter "C" read "G"; and th read with these corrections therein that the sam "both", and ce insert the word "of"; page e said Letters Patent should be e may conform to the record of Signed and sealed this 23rd day of October, A. D. 1928 (Seal) M. J. Moore, Acting Commissioner of Patents. 

